This invention relates to a measuring apparatus for measuring the gas loading of a liquid, particularly of a liquid plastics component, comprising a pump which is disposed in a circuit for the sample quantity and the pump piston of which can be adjusted, with displacement control and by means of an actuating drive, in both directions of travel in the measuring cylinder chamber, comprising a switchable valve device forming a filling and ejection valve for filling the measuring cylinder chamber with the sample quantity to be measured and for carrying away the sample quantity from the measuring cylinder chamber after the measuring operation has been performed, and comprising a pressure measuring device which is associated with the measuring cylinder chamber and with which the pressure of the sample quantity, which is isolated in the measuring cylinder chamber when the valve device is closed, can be measured at different predetermined positions of displacement of the pump piston in order to determine the gas loading. The present invention is further oriented towards a method of determining the gas loading of a liquid, using the measuring apparatus according to the invention in particular.
In technology, and above all in plastics processing technology, it is frequently necessary to measure the gas loading of liquids, e.g. the content of air of liquid plastics components for the production of foamed materials, generally with the aim of being able to operate with a constant gas loading in continuous operation.
Numerous measuring apparatuses have long been known for determining the gas loading of liquids, particularly of liquid plastics components (DE 37 20 904 A1, DE 33 36 037 A1, DE 31 32 597 C2, DE 36 30 209 A1, DE 41 19 966 A1, EP 0 125 541 A2). These known measuring apparatuses generally employ measuring cylinders which are provided with measuring pistons and into which a defined amount of sample is drawn off at intervals from the system conveying the gas-laden liquid. A measurement is then made on this sample, which is isolated from the system in the measuring cylinder. This is generally effected by subjecting the sample quantity, which is loaded with dissolved gas and possibly with free gas also, to a reduced pressure in the closed measuring cylinder chamber, with the release of the gas, by increasing the volume of the measuring cylinder chamber by correspondingly adjusting the piston. The gas loading of the liquid can be calculated from the changes in volume determined and from the changes in pressure of the sample quantity in accordance with known physical relationships (gas law). It is possible to achieve the same result if the measuring sample is measured in the measuring cylinder by compression by means of the measuring piston and/or by a combination of decompression and subsequent compression, with the measurement of pressure under the different test conditions and simultaneous determination of the volume of the measuring space for the different piston positions.
A measuring apparatus for measuring the gas loading of a liquid, particularly a liquid plastics component, comprising a pump which is disposed in a circuit for the sample quantity and the pump piston of which can be adjusted, with displacement control and by means of an actuating drive, in both directions of travel in the measuring cylinder chamber, comprising a switchable valve device forming a filling and ejection valve for filling the measuring cylinder chamber with the sample quantity to be measured and for carrying away the sample quantity from the measuring cylinder chamber after the measuring operation has been performed, and comprising a pressure measuring device which is associated with the measuring cylinder chamber and with which the pressure of the sample quantity, which is isolated in the measuring cylinder chamber when the valve device is closed, can be measured at different predetermined positions of displacement of the pump piston in order to determine the gas loading is known from U.S. Pat. No. 4,376,172. The measuring cylinder chamber of which is connected on its inlet side, via a filling valve, to the system which carries the gas-laden liquid under pressure and which comprises a supply vessel for the gas-laden liquid, and which is connected on its outlet side, via an ejection valve, to a return line which leads back to the supply vessel. Here also, the gas loading is measured by withdrawing a defined amount of sample from the system and feeding it to the measuring cylinder of the measuring pump, followed by making a measurement on the sample with the filling and ejection valves closed, namely with the sample isolated from the system, for which purpose firstly a reduced pressure and then an overpressure is generated in the measuring cylinder by adjusting the piston with control of the displacement thereof. In the course of this procedure, the measured pressure values, as well as the changes in volume of the measuring cylinder chamber which result from the displacement movements of the measuring piston, are processed by an electronic evaluation unit and are utilized for controlling the gas loading of the liquid at a predetermined constant value. After the measuring operation has been performed, the filling valve and the outlet valve are opened again, so that the test sample is returned to the supply vessel again by the pump pressure in the system. In the intervals between the periodic measurements made on samples, the measuring cylinder of the pump is always connected to the pumping system of the installation as a whole.
This invention stems from these known measuring apparatuses, and stems in particular from the apparatus which employs measuring pump according to the aforementioned U.S. Pat. No. 4,376,172. The object of the invention is primarily to fashion this system, particularly the piston pump which is used here as the measuring apparatus for gas loading, without an excessive cost of construction and so that an accurate measurement of the gas loading of liquids, particularly of liquid plastics components for the production of synthetic foam, can be achieved in different operating systems and with different arrangements of the piston pump.
This object is achieved according to the invention by providing the actuating drive of the piston pump as a controlled servomotor together with a transmission gear and by constructing the piston pump at the same time as a feed pump which returns the test sample from the measuring cylinder chamber to the system after the measuring operation has been performed.
With this design of measuring apparatus, and by means of the controlled servomotor and of the associated transmission gear, it is possible to make a very accurate adjustment of the displacement of the pump piston both in the direction of suction and in the direction of compression, and it is consequently possible to make an accurate determination of the change in volume of the measuring cylinder chamber which occurs during the measuring operation, so that the gas content of the test sample can be determined exactly via the displacement-dependent changes in volume and via the pressure values which are measured simultaneously. At the same time, the piston pump according to the invention can operate as a feed pump which pushes the test sample out of the pump cylinder forming the measuring cylinder chamber when the ejection valve is opened after the measurement has been made. Thus,it is also possible to locate the piston pump according to the invention in a separate circuit which is separated from the pumping system of the installation as a whole, and to shut off the measuring cylinder chamber of the piston pump from the pressurised system in the intervals between measuring operations. In this connection, valves which can be switched independently of each other from their closed position into their open position and vice versa are preferably used for the filling valve and the ejection valve which are associated with the piston pump, so that it is possible to fill the measuring cylinder chamber with the test sample, with the filling valve open and the ejection valve closed, and it is possible to eject the test sample after the measuring operation has been performed, with the filling valve closed and the ejection valve open.
In a further advantageous embodiment of the invention, the measuring apparatus which is associated with the piston pump is constructed so that it measures both the pressure of the test sample at different volumes of the measuring cylinder chamber and the temperature of the test sample. The effect of temperature on the gas absorption behavior of the liquid can thereby also be measured metrologically and can be evaluated, e.g. to achieve constant gas loading of the liquid in operation.
Different drive components can be used both for the actuating drive and for the transmission gear of the piston pump according to the invention. A torque motor, the direction of rotation of which can be reversed, preferably an electric torque motor in the form of a stepper motor, can advantageously be used for the servomotor. One advantageous embodiment of the transmission gear consists of a spindle gear, to the axially displaceable spindle nut of which, which is secured against rotation, the pump piston is coupled. The piston pump which is provided according to the invention can be designed as a compact unit. It advantageously possesses a cylinder body which comprises the measuring cylinder chamber and which, as the main body, is attached via a housing body, which is preferably cylindrical and which receives the transmission gear or the spindle gear, to a connection body for the attachment of the servomotor. The various components can advantageously be detachably connected to each other by screwed joints or the like. The aforementioned connection body, to which the servomotor can be joined by a flange, can form a housing for receiving a coupling device which couples the servomotor to the transmission gear or the spindle gear.
One quite particularly advantageous embodiment of the invention, which is of independent patentable importance in combination with a measuring apparatus for measuring the gas loading of a liquid, particularly a liquid plastics component, comprising a pump which is disposed in a circuit for the sample quantity and the pump piston of which can be adjusted, with displacement control and by means of an actuating drive, in both directions of travel in the measuring cylinder chamber, comprising a switchable valve device forming a filling and ejection valve for filling the measuring cylinder chamber with the sample quantity to be measured and for carrying away the sample quantity from the measuring cylinder chamber after the measuring operation has been performed, and comprising a pressure measuring device which is associated with the measuring cylinder chamber and with which the pressure of the sample quantity, which is isolated in the measuring cylinder chamber when the valve device is closed, can be measured at different predetermined positions of displacement of the pump piston in order to determine the gas loading, is obtained if an ultrasonic generator is associated with the measuring cylinder chamber of the measuring apparatus. By means of this ultrasonic generator, ultrasonic vibrations can be generated in the liquid sample which is accommodated in the measuring cylinder chamber whilst the pump piston is either moved in the measuring space in order to reduce the pressure or is held at the measuring pressure. The result of these ultrasonic vibrations is that the gas which is contained in the liquid is expelled considerably more rapidly than is possible solely by means of the reduced pressure which is generated. The time which is necessary to make an accurate measurement on a sample can thus be considerably reduced by means of the reduced pressure generator in combination with the ultrasonic generator. By the generation of ultrasonic waves in the sample, the time from introducing the sample quantity into the measuring cylinder until the instant at which a usable reading is obtained can in some cases be reduced to one quarter of the time which is necessary for a reliable measurement without ultrasound. Thus,the method according to the invention not only enables a particularly accurate determination of the gas loading to be made, but also enables this determination to be made very rapidly.
The inlet and the outlet for filling and emptying the measuring cylinder chamber advantageously lead radially into the latter at the side. The ultrasonic generator can then be disposed at one axial end of the measuring cylinder chamber, which results in a particularly compact arrangement and means that the ultrasonic generator is readily accessible. The latter is preferably replaceably accommodated in an opening in the measuring cylinder chamber, wherein in order to avoid direct contact between the liquid on which a measurement is to be made and the ultrasonic generator it may be advantageous if the opening is closed by a membrane. In this situation, however, it must be ensured that the vibrations generated by the ultrasonic generator are transmitted into the liquid unimpeded by the membrane. A particularly advantageous solution is achieved if the ultrasonic generator is accommodated in a holding flange and is secured thereto by means of fastening screws so that it can easily be replaced if this should become necessary, in the event of a malfunction,for example.
Instead of an ultrasonic wave generator, it is also possible to provide a different type of vibration generator, for example a rotating eccentric or the like, which generates vibrations of lower frequency at the piston pump, which vibrations are transmitted into the liquid sample and thus result in the more rapid expulsion of the gas from the liquid.
The measuring cylinder chamber can comprise at least one connection opening for a pressure and/or temperature sensor on its cylinder wall or on the cylinder body, by means of which sensor the pressures and temperatures in the individual measuring situations can be determined. A closable inspection and/or servicing opening to the measuring cylinder chamber can be provided in the cylinder wall, in order to provide a view into the measuring cylinder chamber through a sight glass or the like or to create an access for cleaning without the entire apparatus having to be completely dismantled.
The measuring apparatus according to the invention can be used particularly advantageously in a system, especially a plastics processing system, in which the measuring cylinder chamber of the piston pump is connected on its inlet side, via the filling valve, as directly as possible and in any event by a line which is as short as possible, to a vessel which contains the gas-laden liquid under an initial pressure and which is provided with a gas supply, and in which the measuring cylinder chamber of the piston pump is connected on its outlet side, via the ejection valve, to the return line to the supply vessel. The circuit for sample measurements can be capable of being shut off by means of an isolating valve from the system which is supplied with the gas-laden liquid from the supply vessel, so that the system pressure is not applied to the inlet side of the piston pump. Moreover, as is known in the art, the system advantageously comprises an evaluation and control device, which is connected via electrical signal and control lines to the measuring device which measures the pressure and which advantageously also measures the temperature of the test sample, and which is connected to the actuating drive and is also advantageously connected to a gas supply valve, which is disposed in a gas supply line leading to the supply vessel and by means of which the gas loading of the liquid situated in the supply vessel is adjusted to a constant set value.
Other features and advantages of the invention follow from the individual subsidiary claims and from the description given below of preferred examples of embodiments which are illustrated in the drawings.